The proposed research is aimed at analyzing mechanisms of human selective attention on both physiological and psychological levels. The broad goals are to identify the neural structures and cortical areas in which stimulus selection takes place in both visual and auditory modalities and to determine the level(s) of perceptual processing at which attention operates. The principal approach is to record event-related brain potentials from multiple scalp sites and utilize source localization techniques including voltage topographic mapping, current source density analyses, and inverse dipole modelling to characterize the underlying neural generators. Proposed experiments in the auditory modality will investigate the neural bases of early channel-selective attention indexed by mid-latency (P20-50) and longer latency (mismatch negativity and Nl) components. Visual experiments will attempt to localize the cortical areas that mediate selective attention to spatial versus non-spatial aspects of visual stimuli. In both modalities, the significance of attention-related ERP changes for perceptual processing will be examined by obtaining "attention operating characteristic" curves for ERP components and measures of detection performance. This research relates to important mental health problems, since disturbances of selective attention are characteristic of many clinical disorders including schizophrenia, attention deficit disorder and learning disability. The ERP studies proposed here should lead to an improved understanding of the basic mechanisms of both normal disordered attention.